1. Field of the Invention
The present invention relate to a liquid crystal display device and a method for producing the same. More specifically, the present invention relates to a liquid crystal display device having a display medium with a structure in which liquid crystal regions are partitioned by a polymeric material and a method for producing the same.
2. Description of the Related Art
As display devices utilizing an electro-optic effect, liquid crystal display devices using nematic liquid crystals have conventionally been used. Examples of such liquid crystal display devices include a twisted nematic (TN) liquid crystal display device and a super-twisted nematic (STN) liquid crystal display device. Liquid crystal display devices using ferroelectric liquid crystal have also been proposed. These liquid crystal display devices include a pair of glass substrates, nematic liquid crystal or smectic liquid crystal provided between the substrates, and two polarizing plates sandwiching the substrates.
Furthermore, as the display devices utilizing an electro-optic effect, liquid crystal display devices utilizing a light scattering phenomenon of liquid crystal, instead of using the polarizing plates, have been known. Such liquid crystal display devices use a dynamic scattering (DS) mode and a phase change (PC) mode.
In recent years, liquid crystal display devices requiring no alignment treatment have been proposed. Such a liquid crystal display device electrically regulates a transparent state and an opaque state by using the birefringence of a liquid crystal. More specifically, in such a liquid crystal display device, the refractive index of liquid crystal molecules with respect to ordinary light is matched with the refractive index of a supporting medium which supports the liquid crystal. Thus, liquid crystal molecules are oriented under the application of a voltage and hence a transparent state is displayed; whereas the orientation of the liquid crystal molecules is disturbed under the application of no voltage and hence a light scattering state is displayed. Japanese National Publication No. 61-502128 discloses a specific method: liquid crystal and photopolymerizable or thermosetting resin are mixed and resin is cured to deposit liquid crystal, whereby liquid crystal droplets are formed in the resin.
Japanese Laid-Open Patent Publication Nos. 4- 338923 and 4-212928 disclose a liquid crystal display device using polarizing plates with improved viewing angle characteristics, i.e., a polymer dispersed liquid crystal device sandwiched with polarizing plates disposed so that the respective polarizing directions cross at right angles (hereinafter, referred to as crossed polarizing plates). These convectional liquid crystal display devices have improved viewing angle characteristics. However, the use of scattering of light for the elimination of polarization makes the brightness of the device xc2xd that of a TN liquid crystal display device. Thus, these conventional devices have not found a wide range of use.
Furthermore, Japanese Laid-Open Patent Publication No. 5-27242 discloses a method for improving viewing angle characteristics by disturbing the orientation of liquid crystal molecules with walls or projections of a polymer to form random domains. However, according to this method, liquid crystal domains are formed at random, a polymeric material enters a pixel portion, and a plurality of disclination lines formed at random between the liquid crystal domains are not eliminated even under the application of a voltage. For these reasons, the conventional liquid crystal display devices have the disadvantage of low contrast, light transmittance under the application of no voltage is low, that is, the black level is not satisfactory under the application of a voltage.
Accordingly, the conventional liquid crystal display devices using polarizing plates have poor viewing angle characteristics and hence are unsuitable for use as a liquid crystal display device for a wide viewing angle. For example, a TN liquid crystal display device undergoes alignment treatment so that liquid crystal molecules rise in the same direction under the application of a voltage. That is to say, the TN liquid crystal display device has a structure in which liquid crystal molecules have an initial orientation of 90xc2x0 twist and rise in one direction at a certain angle, i.e., a pretilt angle. This makes the liquid crystal molecule tilt in the same direction in the case where a gray scale display is conducted to allow the liquid crystal molecules to rise, as shown in FIGS. 51(a) to 51(c). Because of this, as shown in FIG. 51(b), when the liquid crystal molecules are viewed from directions A and B, apparent refractive indices become different. This also makes the difference in contract between directions A and B large, and in some cases, results in an abnormal display such as a change in hue and the reversal of black and white colors.
As described above, the conventional liquid crystal display devices have the disadvantage of poor viewing angle characteristics.
Another method for producing a liquid crystal display device using polarizing plates has been proposed. According to this method, first, a mixture containing liquid crystal and a photopolymerizable material is provided between a pair of substrates. Then, light is irradiated to the mixture to a predetermined pattern through a photomask. At this time, the liquid crystal is phase-separated from the polymeric material in a regulate manner. A shown in FIGS. 52(a) to 52(c), when a voltage is applied to the device thus produced, liquid crystal molecules interact with the polymer and consequently, the liquid crystal molecules rise along walls in each direction. Because of this, apparent refractive indices become nearly the same in directions A and B in FIG. 53(b), improving viewing angle characteristics.
For improving the viewing angle characteristics most effectively, liquid crystal molecules in each pixel should be oriented so as to be symmetric with respect to an axis. However, the axisymmetric orientation requires walls, pillars, or the like of a polymer in the middle of the pixels. This leads to problems during practical use, such as the reduction of liquid crystal regions and decreased light transmittance under the application of no voltage. Furthermore, in this case, disclination lines between the liquid crystal domains cannot be controlled, which makes it impossible to eliminate the disclination lines even under the application of voltage. As a result, the display quality is degraded. Alternatively, the decrease in contrast due to difficulties in eliminating disclination lines degrades the display quality.
The liquid crystal display device of this invention, comprises: two substrates respectively defining a plurality of pixels, each pixel being a display unit, at least one of the substrates being transparent; and a display medium layer formed between the two substrates, having a supporting medium made of a polymeric material and a liquid crystal, the liquid crystal being respectively filled in a plurality of liquid crystal regions being partitioned by supporting walls made of the polymeric material in the supporting medium and each having a size corresponding to a size of each of the plurality of pixel regions, wherein molecules of the liquid crystal filled in the plurality of liquid crystal domain is axisymmetrically oriented in an imaginary plane parallel with a surface of the substrates, and at least one liquid crystal domain is positioned in each of the plurality of the liquid crystal regions.
In one embodiment of the present invention, one liquid crystal domain is positioned in each of the plurality of liquid crystal regions.
In another embodiment of the present invention, a plurality of liquid crystal domains are positioned in each of the plurality of liquid crystal regions, liquid crystal molecules in each domain are axisymmetrically oriented, and the supporting walls made of the polymeric material are present outside of each domain.
In still another embodiment of the present invention, a thin film made of a material selected from the group consisting of an organic material and an inorganic material is provided on surfaces of the two substrates.
In still another embodiment of the present invention, the two substrates are sandwiched between polarizing plates.
In still another embodiment of the present invention, a product xcex94 nxc2x7d of anisotropy of refractive index xcex94 n of the liquid crystal and a cell gap d between the two substrates is in the range of 300 nm to 650 nm.
In still another embodiment of the present invention, a twist angle of the liquid crystal between the two substrates when the liquid crystal is injected therebetween is in the range of 45xc2x0 to 150xc2x0.
In still another embodiment of the present invention, the liquid crystal has a viscosity xcexc of 50 mPaxc2x7s or less at 20xc2x0 C. and dielectric constant anisotropy xcex94 xcex5 of +3(1 kHz) or more.
In still another embodiment of the present invention, the liquid crystal satisfies a condition under which a voltage V10 is 2 volts or less in a voltage-light transmittance characteristic at 25xc2x0 C., in a TN cell when light transmittance of the liquid crystal changes from an initial state to 90%.
In still another embodiment of the present invention, a product xcex94 nxc2x7d of anisotropy of refractive index xcex94 n of the liquid crystal and a cell gap d between the two substrates is in the range of 1000 nm to 1400 nm, and a twist angle of the liquid crystal present in a cell is in the range of 45xc2x0 to 150xc2x0.
In still another embodiment of the present invention, a product xcex94 nxc2x7d of anisotropy of refractive index xcex94 n of the liquid crystal and a cell gap d between the two substrates is in the range of 550 nm to 800 nm, and a twist angle of the liquid crystal present in a cell is in the range of 240xc2x0 to 300xc2x0.
In still another embodiment of the present invention, the supporting walls reach each of the two substrates.
In still another embodiment of the present invention, a center axis of orientation of the liquid crystal domain present in the pixels is orthogonal to at least one of the substrates.
In still another embodiment of the present invention, disclination lines are formed at the periphery of the liquid crystal regions under an application of a voltage.
In still another embodiment of the present invention, the liquid crystal molecules in the liquid crystal regions are axisymmetrically oriented so as to be in parallel with the surface of the substrates, a center axis of an orientation of the liquid crystal domain is aligned in a vertical direction to the substrates, and the polymer material in the supporting walls is symmetrically oriented with respect to the center axis, whereby disclination lines are not formed in the liquid crystal regions under the application of a voltage.
In still another embodiment of the present invention, the liquid crystal molecules in the liquid crystal regions are axisymmetrically oriented so as to be in parallel with the surface of the substrates, a center axis of orientation of the liquid crystal regions is aligned in a vertical direction to the substrates, and the polymer material in the supporting walls is oriented in one direction, whereby disclination lines are not formed in the liquid crystal regions under the application of a voltage.
In still another embodiment of the present invention, a polymer present between the substrates and the liquid crystal in the liquid crystal regions has a pretilt angle axisymmetric with respect to a center axis of orientation of the liquid crystal domain, whereby disclination lines are not formed in the liquid crystal regions under the application of a voltage.
In still another embodiment of the present invention, a black mask is provided on one of the substrates so as to correspond to a center portion of the domains in which the liquid crystal molecules are radially oriented.
According to another aspect of the present invention, the method for producing a liquid crystal display device of this invention comprises the steps of (1) providing a mixture containing a liquid crystalline compound and a photopolymerizable compound between two substrates, at least one of which is transparent; and (2) irradiating light having a predetermined irradiation intensity distribution to the mixture between the two substrates, allowing a phase separation of the mixture involved in polymerization thereof to be effected, and uniformly distributing supporting walls made of a resin and a liquid crystal.
In one embodiment of the present invention, a photopolymerization initiator is added to the mixture.
In another embodiment of the present invention, in step (2), a uniform distribution of the supporting walls and the liquid crystal is determined so as to correspond to an arrangement pitch of a plurality of pixels defined by the two substrates.
In still another embodiment of the present invention, step (2) includes alignment treatment for allowing molecules of the liquid crystal partitioned by the supporting walls to be axisymmetrically oriented in an imaginary plane parallel with a surface of the substrates.
In still another embodiment of the present invention, a light-shielding chip corresponding to a center portion of an axisymmetric orientation of the liquid crystal molecules is formed on either of the two substrates.
In still another embodiment of the present invention, the mixture is irradiated with light having uniform irradiation intensity distribution under a condition that a UV-rays component in a short wave-length region of 300 nm or less is shielded.
In still another embodiment of the present invention, the UV-rays component in a short wavelength region is shielded by using a UV-rays cut filter.
In still another embodiment of the present invention, the UV-rays component in a short wavelength region is shielded by using an inorganic and organic material which makes transmittance of light with a wavelength of 300 nm not more than 10% and transmittance of light with a wavelength of 350 nm at least 40%, assuming that light transmittance with respect to air is 100%.
In still another embodiment of the present invention, while being substantially controlled, at least one of an electric field and a magnetic field is applied to the mixture during light irradiation.
In still another embodiment of the present invention, the electric field is applied by using an electrode for a display.
In still another embodiment of the present invention, light having the predetermined irradiation intensity distribution is formed by using a photomask.
Alternatively, the method for producing a liquid crystal display device of this invention comprises the steps of: injecting a mixture containing a liquid crystalline compound, a photopolymerizable compound, and a liquid crystalline photopolymerizable compound between electrode substrates in a cell, at least one of the substrates being transparent; and irradiating the mixture with light having a uniform irradiation intensity distribution while at least one of an electric field and a magnetic field is applied to the mixture, thereby allowing phase separation involved in polymerization to be effected.
In one embodiment of the present invention, a temperature of the cell during light irradiation is set to be at least a temperature at which the liquid crystalline compound to be used exhibits an isotropic phase, and then the cell is cooled.
In another embodiment of the present invention, the photopolymerizable compound includes a fluorinated compound.
The liquid crystal display device of the present invention has a structure in which a display medium layer is sandwiched between two substrates. For producing such a liquid crystal display device, in the first step, a mixture containing a liquid crystalline compound, a photopolymerizable compound, and a photopolymerization initiator is provided between the two substrates. In the second step, the mixture is irradiated with light having a predetermined irradiation intensity distribution, thereby allowing phase separation involved in polymerization of the mixture to be effected. In this way, the display medium layer having a structure in which supporting walls made of the polymer (resin) and liquid crystal are uniformly distributed is obtained.
Molecules in the liquid crystal filled in a plurality of liquid crystal regions in the display medium layer are axisymmetrically oriented in an imaginary plane parallel with a surface of the substrates. Furthermore, at least one liquid crystal domain is positioned in each of the plurality of liquid crystal regions.
When the angle and direction, in which the liquid crystal display device of the present invention are observed from outside, are changed, the dependence of display contrast on a viewing angle can be eliminated because of the axisymmetric orientation of the liquid crystal molecules. Also, the axisymmetric orientation of the liquid crystal molecules prevents the disclination lines from being formed in the liquid crystal region, and thus the display quality is remarkably improved.
According to the present invention, the product xcex94 nxc2x7d of anisotropy of refractive index xcex94 n of the liquid crystal material and a cell gap d (distance between substrates sandwiching a display medium) is set to be in the range of 300 to 650 nm, and the twist angle of liquid crystal between the substrates is set to be in the range of 45xc2x0 to 150xc2x0 when the liquid crystal is injected therebetween. Because of this, the light transmittance of the display device can be optimized, and the light transmittance of the liquid crystal display device can be remarkably improved.
Thus, the invention described herein makes possible the advantages of (1) providing a liquid crystal display device with markedly improved viewing angle characteristics and display quality; and (2) a simplified method for producing a liquid crystal display device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.